In the conventional display device, the backlight module generally includes a light source and a reflective sheet, a light guide plate, a lower diffusion sheet, a lower prism sheet, an upper prism sheet and an upper diffusion sheet (or a brightness enhancement film) arranged from bottom to top. The light guide plate is the main component of the backlight module to guide the beam to the display panel. When the beam emitted from the light source enters the light guide plate, the beam is transmitted toward the other end of the light guide plate by the principle of total reflection, and during the duration, the beam is only transmitted in the light guide plate and cannot be emitted from the light emitting surface of the light guide plate. In order to allow the beam to be emitted from the light emitting surface of the light guide plate, a microstructure is disposed on the bottom surface of the light guide plate. When the beam is transmitted to the bottom of the light guide plate and touches the microstructure, the beam reflected by the microstructure will diffuse at various angles, so that the total reflection condition and the interference of light are destroyed and the beam can be emitted from the light emitting surface of the light guide plate.
However, after the beam emitted from the light source enters the light guide plate and then is emitted from the light guide plate, the beam will be reflected back and forth between the prism sheet and the reflective sheet. The more the number of reflections between the prism sheet and the reflective sheet, the more the attenuation of energy of the beam, which may lead to an overall luminance decrement. Specifically, when the beam penetrates the bottom surface of the light guide plate, the reflected light reflected by the reflective sheet is transmitted back to the light guide plate and is emitted to the lower prism sheet through the light emitting surface of the light guide plate. If the angle of the reflected light cannot match the light emit angle of the lower prism sheet, the reflected light will be reflected back into the light guide plate again. If the aforementioned reflection occurs repeatedly, the loss of light energy will increase. In general, in order to increase the luminance, in addition to dispose microstructures having different densities or different shapes on the light guide plate, the number of light sources or the current of the light source can also be increased, but the enhance effect is limited and the cost maybe increased. Therefore, how to reduce the number of times that the beam is reflected back and forth between the prism sheet and the reflective sheet, thereby improving the above-mentioned problem and allowing the beam can be effectively derived from the prism sheet is the focus of attention of the relevant personnel in the field.
The information disclosed in this “BACKGROUND OF THE INVENTION” section is only for enhancement understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Furthermore, the information disclosed in this “BACKGROUND OF THE INVENTION” section does not mean that one or more problems to be solved by one or more embodiments of the invention were acknowledged by a person of ordinary skill in the art.